In the drilling of a wellbore through a subterranean formation by rotary drilling techniques, conventionally a drilling fluid is circulated from a well head, which may be at the surface of the earth, down a drill string having a drill bit on the lower end thereof and through ports provided in the drill bit to the well bottom and thence back to the well head e.g. the surface through the annulus formed about the drill string. Commonly, drilling fluids are employed that are either oil or water based. These fluids are treated to provide desired rheological properties which make the fluids particularly useful in the drilling of wellbores.
A problem often encountered in the drilling of a well bore is the loss of unacceptably large amounts of drilling fluid into the subterranean formation penetrated by the wellbore. This problem is often referred to generally as “lost circulation”, and the formation zones into which the drilling fluid is lost are often referred to as “lost circulation zones” or “thief zones”. Various causes may be responsible for the lost circulation encountered in the drilling of a wellbore. For example, a formation penetrated by the wellbore may exhibit unusually high permeability or may contain fractures or crevices therein. In addition, a formation may simply not be sufficiently competent to support the hydrostatic pressure applied by the drilling fluid and may break down under this hydrostatic pressure and allow the drilling fluid to flow thereinto.
In an attempt to overcome these problems and prevent loss of drilling fluid from the wellbore, a device was proposed in WO2005/024178, the disclosure of which is herein incorporated by reference. The proposed device was for lining the wall of a wellbore as it is being drilled through a subterranean formation using a drill string having a drill bit on the lower end thereof, the device comprising:                (a) a receptacle for a cylindrically gathered pack of flexible tubing and        (b) a radially expandable locking means having means for directly or indirectly attaching a first end of the tubing of the gathered packwherein        (ii) the receptacle for the gathered pack of flexible tubing is capable of being supported around the outside of the drill string at or near the lower end thereof on a plurality of bearings thereby allowing the receptacle to remain stationary while the drill string is rotated; and        (iii) the radially expandable locking means is capable of being expanded against the wellbore wall thereby locking the first end of the flexible tubing in place in the wellbore such that in use movement of the drill string through the wellbore as it is being drilled causes the flexible tubing to be withdrawn from the cylindrically gathered pack and to be turned inside out thereby providing a liner for the wellbore wall with the outer surface of the tubing of the cylindrically gathered pack forming the inner surface of the liner.        
WO 2005/024178 also describes that device with the gathered pack of flexible tubing as well, and a method of sealing the wall of a well bore using the device. In a conventional drilling operation the drill fluid containing the drill cuttings in the entrained cuttings stream returns from the drill bit to the well head up the annulus between drill string and well bore or casing. But according to WO2005/024178 the entrained cuttings stream is prevented in this case from passing directly back to the well head over the outside of the drill string owing to the fluid barrier imposed by the withdrawn flexible tubing. Accordingly, a fluid by-pass is provided for the entrained cuttings stream. For example, the interior of the drill string may be provided with at least one conduit having an inlet below the cylindrical receptacle for the gathered pack of tubing and an outlet above the cylindrical receptacle such that the entrained cuttings stream passing through the conduit bypasses the cylindrical receptacle. Alternatively, the cylindrical receptacle may itself be provided with a fluid by-pass. The entrained cuttings stream then flows to the surface through the annulus formed about the drill string in the standard manner.
However the fluid barrier provided by the withdrawn flexible tubing still creates a problem, namely that the tubing, as the sheath, can peel off the well bore wall back to an expanded locking means.